1. Field of the Invention
The present invention relates to a micro-lens built-in vertical cavity surface emitting laser (VCSEL) in which a micro-lens is formed on a laser beam emitting surface of the VCSEL, and more particularly, to a micro-lens built-in VCSEL capable of emitting a parallel light beam.
2. Description of the Related Art
In general, VCSELs emit a light beam in a direction of a semiconductor material layer stack, and thus it is easy to optically combine VCSELs with another optical element and to assemble the VCSELs into an apparatus. In addition, the VCSELs can also be manufactured to have a two-dimensional array structure. Thus, the VCSELs can be widely applied as light sources in optical transmission systems for applications such as optical communications or interfacing using optical signals, and in optical recording/reproducing optical heads.
Referring to FIG. 1, a conventional VCSEL includes a substrate 10, a lower reflector 11, an active layer 12, a high-resistance region 13 and an upper reflector 14 which are sequentially stacked on the substrate 10, an upper electrode 16 formed on a portion of the upper reflector 14 excluding a window 18 through which a laser beam is emitted, and a lower electrode 17 formed underneath the substrate 10.
Each of the lower reflector 11 and the upper reflector 14 is a distributed Bragg reflector (DBR) which is formed by alternately stacking semiconductor material layers having different refractive indexes, and having an opposite doping type. That is, the substrate 10 and the lower reflector 11 are doped with the same n-type impurities and the upper reflector 14 is doped with p-type impurities.
The high-resistance region 13 guides the flow of current passed through the upper and lower electrodes 16 and 17 into a center of the active layer 12. The active layer 12 is a region where light is generated by a combination of holes and electrons from the upper and lower reflectors 14 and 11, where the combination of holes and electrons is induced by a current applied across the upper and lower electrodes 16 and 17. Light generated in the active layer 12 is reflected repeatedly between the upper and lower reflectors 14 and 11 and only a light having a wavelength in accordance with the resonance condition remains, and is emitted through the window 18.
In the conventional VCSEL having the aforementioned structure, the laser beam emitted through the window 18 has a predetermined radiation angle. Thus, when such a conventional VCSEL is adopted as a light source, for example, for an optical transmission system using an optical cable, a condensing lens for condensing a divergent laser beam emitted from the VCSEL is required between the VCSEL and an input terminal of the optical cable, so as to efficiently couple the laser beam emitted from the VCSEL to the optical cable.
As another example, when the above-mentioned conventional VCSEL is adopted as the light source for an optical head in an optical recording/reproduction apparatus which records information on or reproduces information from a recording medium such as an optical disc in a non-contact manner, the optical head needs a collimating lens for condensing a divergent laser beam emitted from the conventional VCSEL into a parallel laser beam.
Briefly, because the conventional VCSEL emits the divergent laser beam through the window 18, a separate condensing lens or collimating lens is needed at the light emitting side of the VCSEL so as to construct an optical system with the VCSEL. Thus, when constructing an optical system, a number of parts required increases and there is a need to align the lens with a central optical axis of the laser beam emitted from the VCSEL, making an optical alignment structure complicated.